1. Field of the Invention
The present invention relates to a driving circuit of an ink jet print head in a printing device, and more particularly, to a driving circuit that balances thermal energy among heating elements of ink jet print cells.
2. Description of the Prior Art
Please refer to FIG. 1. FIG. 1 is a diagram of a prior art ink jet print head 70. The ink jet print head 70 comprises an ink tank 72, a plurality of channels 74, and a plurality of ink jet cells 76. The ink tank 72 connects to the plurality of ink jet cells 76 through the plurality of channels 74. Ink in the ink tank 72 can flow into the ink jet cells 76 through the channels 74. A heating resistor 78 is installed alongside each inkjet cell 76. The heating resistor 78 heats up ink in the ink jet cells 76. The plurality of heating resistors 78 form a heating circuit 60, as shown in FIG. 2. When the heating resistor 78 has energy greater than a threshold, bubbles 80 are generated in the ink. The bubbles force ink drops to jet from the nozzles 82 onto the medium (such as paper) to perform printing. However, the amount of ink jetted out is related to the energy supplied by the heating resistors 78. So, if higher energy is supplied, larger ink drops are jetted out and larger ink spots are formed on the medium. If lower energy is supplied, smaller ink drops are jetted out and smaller ink spots are formed on the medium. If the sizes of the ink drops are not uniform or within a limited range, the printing quality is low. Therefore, the energy generated by the heating resistors 78 should be higher than the threshold so as to jet ink drops, and should also be maintained within a limited range so as to form ink drops of substantially equal sizes.
FIG. 2 is a diagram of a prior art ink jet print head driving circuit 10. The driving circuit 10 comprises a row driving module 20 and a column driving module 40. The row driving module 20 receives row data 30 and passes four row control signals R1, R2, R3, R4 to the heating circuit 60 in the ink jet print head. The column driving module 40 receives column data 50 and passes four column control signals C1, C2, C3, C4 to the heating circuit 60 in the ink jet print head. The row driving module 20 comprises a shift register 22, a latch circuit 24, and a starter 27. The column driving module 40 comprises a shift register 42, a latch circuit 44, and a starter 47. The row driving module 20 and the column driving module 40 use a common clock signal 32, a latch signal 34, and a start signal 39.
The shift registers 22 and 42, controlled by the clock signal 32, receiving binary printing data from the printing device. Then, the latch circuits 24 and 44 latch and store the printing data according to the latch signal 34. The starters 27 and 47 are composed of a plurality of AND gates 37. Each of the plurality of AND gates 37 is connected at one input to an output of a corresponding latch circuit 24, 44. Another input of the AND gate 37 is connected to the start signal 39. According to the start signal 39 and content of the latch circuits 24, 44, the starters 27 and 47 cause the heating circuit 60 in the ink jet print head to start to heat the plurality of ink jet cells. The heating circuit 60 comprises a plurality of row and column data lines arranged in an array. Each row data line and column data line is connected by a heating resistor and a transistor switch, which are respectively controlled by row control signals R1, R2, R3, R4 and column control signals C1, C2, C3, C4. The row control signals R1, R2, R3, R4 are respectively connected to the drains of the transistor switches via resistors, and the column control signals C1, C2, C3, C4 are respectively connected to the gates of the transistor switches. When a specific column and a specific row data line are activated at the same time, the transistor corresponding to the activated row and column data lines conducts, so that current flows through the corresponding heating resistor, and the corresponding ink jet cell jets ink drops.
FIG. 3 is a timing diagram of a prior art ink jet print head driving signal. FIG. 3 illustrates the method of driving the prior art ink jet print head. Between times T0 and T1, four row data 30 and four column data 50 are sequentially input to the shift registers 22 and 42, according to the clock signal 32. When a pulse is generated in the latch signal 34, binary bits of the four row data 30 and the four column data 50 are respectively latched and stored in the latch circuits 24 and 44. The row data 30 and the column data 50 now appear at one input of the AND gates 37 of the starter 27. Between times T1 and T2, a pulse is generated in the start signal 39. Thus, according to the data appearing at the inputs of the AND gates 37 of the starter 27, the outputs of the AND gates 37 go high. For example, if between times T0 and T1, the row data 30 (R1,R2,R3,R4) equals to (1, 0, 0, 0), and the column data 50 (C1,C2,C3,C4) equals to (1, 0, 1, 0), then between times T1 and T2, when the pulse of the start signal 39 generates, the row data line RI and the column data lines C1 and C3 are activated. Therefore, the transistors 62 and 64 conduct, causing current to pass through the heating resistors 66 and 68, so that the corresponding ink jet cells are heated and jet ink. Please note that, because other un-activated transistors do not conduct, current does not pass through the corresponding heating resistances, and the corresponding ink jet cells are not heated.
The size of the ink spot jetted from the ink jet cell is an important factor influencing printing quality. The size of the ink spots is related not only to the energy supplied by the heating resistors, but is also related to whether the ink jet cells have been heated in a previous time. More specifically, if an ink jet cell has been heated to jet ink recently, energy accumulation results in jetting larger ink spots in a new ejection. In other words, if heating a previously unheated ink jet cell and a previously heated ink jet cell with a same energy, ink spots of the former are smaller, and ink spots of the latter are larger. Therefore, if heating the ink jet print head with the prior art driving circuit, the jetted ink drops may have varying sizes, which results in poorer printing quality.
It is therefore an objective of the present invention to provide a driving circuit in a printing device that drives heating resistors in a balanced way, so as to improve uniformity of ejected ink spots.
Briefly, the claimed invention provides a driving circuit of an inkjet print head in a printing device. The ink jet print head has a plurality of ink jet cells and corresponding heating elements. Each ink jet cell contains ink and has a nozzle. The driving circuit selectively drives the heating elements to provide energy to the corresponding ink jet cells and to heat the ink jet cells according to printing data from the printing device. The printing data determines whether or not the inkjet cells, and corresponding nozzles, should jet ink. When supplied energy is greater than a threshold, ink drops are jetted from the nozzles onto the medium. The driving circuit has a shift register, a latch circuit, and a driving signal generator. The driving signal generator provides a first driving signal to a first set of nozzles that are expected to jet ink. The first driving signal drives a corresponding first set of heating elements of the first set of nozzles with an energy greater than the threshold to heat a corresponding first set of printing cells, so that ink is jetted from the first set of nozzles. The driving signal generator provides a second driving signal to a second set of nozzles that are expected not to jet ink. The second driving signal drives a corresponding second set of heating elements with an energy less than the threshold, so that a corresponding second set of ink jet cells are heated without jetting ink drops. In this way, the thermal accumulation conditions of different ink jet cells are similar, and the ink jet cells are thus capable of jetting ink drops of uniform sizes to achieve better printing quality.
These and other objectives and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.